Sunbirds suck their nectar, in dramatic contrast to hummingbirds, which sop it up
Sunbirds employ tongue suctioning, which is unique among vertebrates.
University of California - Berkeley
video:
A sunbird sticks its tongue into an artificial flower (top) and uses tongue suctioning to draw out nectar (bottom).
view moreCredit: David Cuban, Brown University
While we often think of hummingbirds as sucking nectar from flowers, they’re not sucking the way we suction juice through a straw — they’re really sponging up nectar with their tongues and squeezing the juice into their mouths by compressing their tongues with their beaks.
Humans are naturally able from birth to use mouth suction to draw in liquid, but it’s not easy if you don’t have lips to create an air-tight seal, and few animals besides mammals have lips.
But a new study by current and former University of California, Berkeley biologists found that sunbirds, the African and Asian counterparts of the nectar-sipping hummingbirds of the Americas, do use suction to slurp nectar. They’re the first animals known to employ their tongues to suction up liquids.
The results highlight the fact that nature often finds different solutions to similar problems — in this case, how to use a long, tubular and often curved beak to extract sustenance from deep within a flower. It’s referred to as convergent evolution.
“It's just a really amazing example of the power and beauty of convergent evolution, where in nature we have two organisms filling the same ecological role, but when you look in detail, they're achieving that outcome in two completely different ways,” said Rauri Bowie, UC Berkeley professor of integrative biology and a study author. “In our case, we're seeing a mechanism that is completely novel in vertebrates and a remarkable example of innovation.”
The proof comes from experiments conducted in Africa and Indonesia using high-speed cameras installed adjacent to 3D printed artificial flowers, plus microCT scans of sunbirds obtained in UC Berkeley’s Museum of Vertebrate Zoology (MVZ). The results were reported last month in the journal Current Biology, in a paper led by first author and Berkeley alum David Cuban, now a postdoctoral fellow at Brown University.
“I am fascinated by the phenomena of convergent evolution,” Cuban said. “Hummingbirds and sunbirds — and some other nectar-feeding birds — have similar morphology, coloration, behavior and ecological niches, but once we zoom in on something specific, in this case their feeding mechanism, we find that they use completely distinct mechanisms.”
Lapping, licking and sucking
Most vertebrates use lapping or licking to take in liquids with their tongues (think dogs and cats). But a few animals employ suction. Fish, for example, inflate their mouths to suction in food. Butterflies use a muscular pump to suction up nectar and pollen. Pigeons suction water from pools, though only by submerging their beaks in the liquid and using their tongue as a piston. These options are not available to nectar feeders, who stick their tongues into the sweet liquid.
For nectar-eating birds — nectarivores — it was thought until recently that they used capillary action to passively tap a pool of nectar. Surface tension was thought to pull liquid into their tube-like beaks or tongues.
But many biologists doubted this, because using capillary action is a slow way to take in calories. For frenetic fliers like hummingbirds and sunbirds, this would not provide sufficient energy.
“When you think about it, there's no way they would ingest enough nectar, given that sunbirds and hummingbirds have this incredibly beautiful ornamentation that they're very actively displaying, using a high calorie intake to fuel their lifestyles,” Bowie said.
Nearly 10 years ago, former UC Berkeley Miller Postdoctoral Fellow Alejandro Rico-Guevara demonstrated with high-speed video that hummingbirds don’t use capillary action. Instead, they compress their tongues before sticking them into a pool of nectar. As the tongue expands, nectar fills the pores as if it were a sponge. As the birds retract the tongue, they squeeze it between the upper and lower bill, wringing out the nectar like the mangle of an old-fashioned washing machine. Hummingbirds do this repeatedly, and it’s about 10 times faster than using capillary action.
Cuban, a former UC Berkeley undergraduate and then master’s student in mechanical engineering, doubted that sunbirds used capillary action either, and set out to prove it. He had become fascinated by the convergent evolution of nectar-feeding birds during an ornithology class taught by Bowie and teaching assistant Cynthia Wang-Claypool, then a Berkeley graduate student. Seeing hummingbird and sunbird specimens laid out next to one another in the MVZ, he said, “I knew I had to look into the convergent evolution of nectar-feeding birds, and I wanted to use my background in mechanical engineering to do so.”
One tip-off that sunbirds were not feeding by capillary action came from early high-speed video analyses. Cuban saw bubbles around the tongue, which would interfere with surface tension, though not suction. Sunbirds also keep their beaks slightly open when drinking nectar, whereas hummingbirds do not.
3D-printed flowers
Working as a doctoral student with Rico-Guevara at the University of Washington in Seattle, he traveled to South Africa and Sulawesi in Indonesia to conduct experiments with seven sunbird species, using high-speed cameras to film birds visiting 3D-printed fake flowers filled with sugar water. He modified techniques that Rico-Guevara had originated to study hummingbird feeding, initially in his native Colombia.
Cuban discovered that sunbirds in Asia, where they originated, and in Africa, which they subsequently colonized, use the same tongue technique to draw in nectar. Their tongues have a V-shaped trough at its base. As the bird sticks its tongue into a pool of nectar, it presses its base against the top beak, creating an air-tight seal. As the bird gradually pulls its tongue back in, this creates suction that draws in liquid via the tongue groove. When the seal breaks, the bird swallows the nectar.
“Pushing the base of the tongue against the top of the beak — that's what is really creating that hermetic seal,” Bowie said. “It's the interaction between the tongue and the beak that creates that negative pressure.”
MicroCT scans by Wang-Claypool, now with the Museum of Comparative Zoology at Harvard University, provided anatomical confirmation.
“They provided the evidence we needed that the structure of the tongue differs from hummingbirds, with sunbirds having special flexible flaps at the base of the tongue so that when the tongue pushes up against the top of the beak, it generates a tight seal,” Bowie said.
The researchers continue to explore the differences between sunbirds and hummingbirds and convergent evolution among nectar eaters — a lifestyle that evolved at least 30 times among animals, Bowie said. A native of South Africa, Bowie started working on sunbirds as part of his Ph.D. thesis at the University of Cape Town and has continued to explore the diversification and evolutionary ecology of these remarkable birds over the 20 years he has been a member of the Berkeley faculty.
“I'm interested in nectarivory as a lifestyle, looking at it from the point of view of the diversification of these species, how they've adapted to different kinds of habitats, including the extensive radiation by both sunbirds and hummingbirds into mountains,” he said.
“By studying the physical interactions, or biomechanics, of organisms we can better understand how the immutable laws of physics are shaping the many diverse adaptations found across the tree of life,” Cuban said.
The work was funded in part by grants from the National Science Foundation (DEB 1457845). Co-authors with Cuban, Bowie, Rico-Guevera and Wang-Claypool are Yohanna Yohanna of the National Research and Innovation Agency of Indonesia, Colleen Downs and Steven Johnson of the University of KwaZulu-Natal in South Africa and Fabian Brau of the Free University of Brussels in Belgium.
A greater double-collared sunbird (Cinnyris afer), one of the South African sunbirds studied.
Credit
Keith Barnes
CT scans reveal striking bill and tongue differences between the olive sunbird (Cyanomitra olivacea, top) and Anna’s hummingbird (Calypte anna, bottom). Cross sections at several places along the bill show that, from base to tip, the sunbird tongue shifts from a U-shaped trough to a closed tube, while the hummingbird tongue transforms from two solid rods into twin hollow tubes. The U-shaped base of the sunbird tongue interacts with the upper bill to create the suction the birds use to sip nectar from flowers.
Credit
Cynthia Wang-Claypool/UC Berkeley
Journal
Current Biology
Method of Research
Experimental study
Subject of Research
Animals
Article Title
Divergent nectar-feeding mechanisms evidenced by intralingual suction in sunbirds
